feat(FN-5252): update done-card files chip display logic

Adds a new files chip on done-state TaskCards showing insertion/deletion counts, backed by new types (`filesChanged`, `insertions`, `deletions`) in core; architecture docs updated to reflect the change. The component implementation is a net reduction of 20 lines, with corresponding test coverage add

Fusion-Task-Id: FN-5252

docs/architecture.md

@@ -1384,13 +1384,16 @@ Done-task file-count surfaces intentionally distinguish three data sources:
 
 1. **`/api/tasks/:id/diff` (lineage union, authoritative landed diff)**
    - This route aggregates the task's landed lineage and returns `stats.filesChanged` plus the file list used by the Changes tab.
-   - Task cards and done-task diff views should treat this as the canonical "files changed" source.
+   - Done-task cards and diff views should treat this as the canonical "files changed" source.
 2. **`task.mergeDetails.filesChanged` / `insertions` / `deletions` (final-commit shortstat)**
    - These fields describe only the recorded final merge/squash commit shortstat.
-   - In multi-commit lineages this can undercount the full landed diff and is therefore labeled as commit-level metadata (for example, "Files in merge commit" / "Final commit summary").
-3. **`task.modifiedFiles` (execution-time worktree snapshot)**
+   - On done cards, `mergeDetails.filesChanged` is only a transient loading placeholder until `/api/tasks/:id/diff` resolves.
+3. **`task.mergeDetails.landedFiles` (recorded committed file list)**
+   - When live diff stats are unavailable, done-task cards may fall back to the recorded landed file list length.
+   - This remains committed-diff metadata; transient executor worktree captures are not surfaced as a done-card files chip.
+4. **`task.modifiedFiles` (execution-time worktree snapshot)**
    - Captured in the executor worktree during implementation (`git diff <base>..HEAD` snapshot), before final merge outcomes are known.
-   - Can include transient/superset paths that did not land; UI labels this as "files touched during execution" rather than "files changed" for done tasks.
+   - Can include transient/superset paths that did not land; done-task cards must not use it for the files-changed chip.
 
 **FN-4647 decision:** `mergeDetails` shortstat fields remain commit-level metadata. No additional persisted lineage-level summary field is introduced at this time; done-task landed totals continue to be served live via `/api/tasks/:id/diff`.
 

packages/core/src/types.ts

@@ -1573,10 +1573,10 @@ export interface Task {
    * task worktree (`TaskExecutor.captureModifiedFiles`).
    *
    * This may be a stale/transient superset of files that actually landed after
-   * merge resolution or follow-up commits. UI surfaces must label this as
-   * "files touched during execution" (never landed "files changed" for done
-   * tasks). The authoritative landed diff for done tasks is
-   * `/api/tasks/:id/diff`.
+   * merge resolution or follow-up commits. Done-task cards must not use this
+   * field for their files-changed chip; the authoritative landed diff comes
+   * from `/api/tasks/:id/diff`, with `mergeDetails.landedFiles` as committed
+   * metadata fallback when live stats are unavailable.
    */
   modifiedFiles?: string[];
   /** Opt out of the squash file-scope invariant for this task. */

packages/dashboard/app/components/TaskCard.tsx

@@ -1522,11 +1522,8 @@ function TaskCardComponent({
     }
 
     if (task.column === "done") {
-      // Per FN-4527/FN-4647: /api/tasks/:id/diff is authoritative for done-task
-      // landed file counts. mergeDetails.filesChanged can be stale after
-      // rebase-and-push (FN-4526), so only use it as a transient loading
-      // placeholder. Executor-captured modifiedFiles are labeled as
-      // "touched during execution" and never as landed "files changed".
+      // Done cards only display committed diff counts from authoritative lineage
+      // stats or recorded landed files; transient execution-touched files are not shown.
       let displayCount: number | undefined;
       if (diffStats) {
         const landed = task.mergeDetails?.landedFiles;
@@ -1537,7 +1534,7 @@ function TaskCardComponent({
       } else if (diffLoading) {
         displayCount = task.mergeDetails?.filesChanged ?? undefined;
       } else {
-        displayCount = undefined;
+        displayCount = task.mergeDetails?.landedFiles?.length;
       }
       if (displayCount != null && displayCount > 0) {
         return (
@@ -1552,25 +1549,6 @@ function TaskCardComponent({
           </button>
         );
       }
-
-      const landedFallbackCount = task.mergeDetails?.landedFiles?.length;
-      const modifiedCount = landedFallbackCount && landedFallbackCount > 0
-        ? landedFallbackCount
-        : task.modifiedFiles?.length;
-      if (!diffLoading && (modifiedCount ?? 0) > 0) {
-        return (
-          <button
-            type="button"
-            className="card-session-files"
-            onClick={handleOpenFiles}
-            disabled={!onOpenDetailWithTab}
-            title="Captured from worktree during execution; may not match the landed diff. Open the task to view the recorded merge."
-          >
-            <Folder size={12} />
-            <span>{modifiedCount} {modifiedCount === 1 ? "file" : "files"} {landedFallbackCount ? "in merged commit" : "touched during execution"}</span>
-          </button>
-        );
-      }
     }
 
     return null;

packages/dashboard/app/components/__tests__/TaskCard.test.tsx

@@ -3135,7 +3135,7 @@ describe("TaskCard", () => {
     expect(filesChangedButton).toBeNull();
   });
 
-  it("prefers landedFiles fallback label for done tasks when lineage stats are unavailable", () => {
+  it("prefers landedFiles fallback files-changed label for done tasks when lineage stats are unavailable", () => {
     const onOpenDetailWithTab = vi.fn();
     useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });
 
@@ -3144,7 +3144,6 @@ describe("TaskCard", () => {
         task={makeTask({
           column: "done",
           mergeDetails: { landedFiles: ["a.ts", "b.ts"] },
-          modifiedFiles: ["a.ts", "b.ts", "c.ts", "d.ts", "e.ts", "f.ts"],
         })}
         onOpenDetail={noop}
         addToast={noop}
@@ -3152,7 +3151,7 @@ describe("TaskCard", () => {
       />,
     );
 
-    const landedButton = screen.getByRole("button", { name: "2 files in merged commit" });
+    const landedButton = screen.getByRole("button", { name: "2 files changed" });
     expect(landedButton).toBeDefined();
 
     fireEvent.click(landedButton);
@@ -3160,11 +3159,10 @@ describe("TaskCard", () => {
     expect(onOpenDetailWithTab.mock.calls[0]?.[1]).toBe("changes");
   });
 
-  it("shows execution-touched fallback label for done tasks when lineage stats are unavailable", () => {
-    const onOpenDetailWithTab = vi.fn();
+  it("hides the done-task file chip when only execution-touched modifiedFiles exist", () => {
     useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });
 
-    render(
+    const { container } = render(
       <TaskCard
         task={makeTask({
           column: "done",
@@ -3172,17 +3170,13 @@ describe("TaskCard", () => {
         })}
         onOpenDetail={noop}
         addToast={noop}
-        onOpenDetailWithTab={onOpenDetailWithTab}
+        onOpenDetailWithTab={vi.fn()}
       />,
     );
 
-    const touchedButton = screen.getByRole("button", { name: "6 files touched during execution" });
-    expect(touchedButton).toBeDefined();
-    expect(screen.queryByText(/\d+ files? changed/i)).toBeNull();
-
-    fireEvent.click(touchedButton);
-    expect(onOpenDetailWithTab).toHaveBeenCalledTimes(1);
-    expect(onOpenDetailWithTab.mock.calls[0]?.[1]).toBe("changes");
+    expect(screen.queryByText(/touched during execution/i)).toBeNull();
+    expect(screen.queryByRole("button", { name: /files changed/i })).toBeNull();
+    expect(container.querySelector(".card-session-files")).toBeNull();
   });
 
   it("prefers lineage files-changed stats over stale execution-touched modifiedFiles for done tasks", () => {
@@ -3215,7 +3209,26 @@ describe("TaskCard", () => {
     );
 
     expect(screen.getByRole("button", { name: "4 files changed" })).toBeDefined();
-    expect(screen.queryByText("10 files touched during execution")).toBeNull();
+    expect(screen.queryByText(/touched during execution/i)).toBeNull();
+    expect(screen.queryByText(/in merged commit/i)).toBeNull();
+  });
+
+  it("uses singular 'file changed' grammar for landedFiles-only done tasks", () => {
+    useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });
+
+    render(
+      <TaskCard
+        task={makeTask({
+          column: "done",
+          mergeDetails: { landedFiles: ["a.ts"] },
+        })}
+        onOpenDetail={noop}
+        addToast={noop}
+        onOpenDetailWithTab={vi.fn()}
+      />,
+    );
+
+    expect(screen.getByRole("button", { name: "1 file changed" })).toBeDefined();
   });
 
   it("hides done-task file chip when lineage stats are unavailable and no execution fallback exists", () => {